First install certbot on your system to accquire a free SSL certificate.

apt-get install python-certbot-apache

To deploy the free SSL certificate for the virtual host on the unit, run the following command:

certbot --apache

Running cerbot will also modify the virtual host .conf to load the respective certificate. To set up automatic renewal, create a cron job (without user input), for example, on 1:00 every day, The cron job will check the expiration status of all certificates loaded on the system, and only update the certificate when it is about to expire.

Alternatively, please visit sslforfree.com and follow its instruction to download and setup SSL for your domain. The certificate is valid for 90 days and thus must be renewed in 90 days. The screenshots below illustrate the step-by-step procedure.